The present invention generally relates to devices and methods for measuring properties of fluids. More particularly, this invention relates to a microfluidic device having a resonating tube capable of sensing the volume of a gas present as bubbles in a liquid flowing through the tube, or the flow rate and/or density of a gas or gas mixture flowing through the tube.
Fluid delivery devices capable of precise measurements find use in a variety of industries, including medical treatment systems such as drug infusion and anesthesia, energy and fuel systems such as fuel cells, and consumer goods. Various types of flow rate and concentration sensors have been proposed, including electrolytic, refractometer, ultrasonic, electrochemical, electromagnetic, and electromechanical sensors. An example of the latter is a Coriolis-based microfluidic device disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al., whose contents relating to the fabrication and operation of a Coriolis-based sensor are incorporated herein by reference.
A microfluidic device 10 of a type disclosed by Tadigadapa et al. is represented in FIGS. 1 and 2. The device 10 is shown as including a micromachined tube 14 extending from a base 28 on a substrate 12, with a freestanding portion 16 of the tube 14 suspended above a surface 18 of the substrate 12. Drive and sensing electrodes 22 and 24 are located on the substrate surface 18 beneath the freestanding portion 16 of the tube 14, and bond pads 32 (only one of which is shown) are provided for transmitting input and output signals to and from the device 10. The drive electrode 22 can be, for example, capacitively coupled to the tube 14 for capacitively (electrostatically) driving the freestanding portion 16 at or near resonance, while the sensing electrodes 24 sense (e.g., capacitively, optically, etc.) the deflection of the resonating tube 14 relative to the substrate 12 and provide feedback to enable the vibration frequency induced by the drive electrode 22 to be controlled with appropriate circuitry.
With a fluid entering the device 10 through an inlet port 26 and flowing through an internal passage 20 within the tube 14, the freestanding portion 16 can be vibrated at or near resonance by the drive electrode 22 to ascertain certain properties of the fluid, such as flow rate and density, using Coriolis force principles. In particular, as the freestanding portion 16 is driven at or near resonance by the drive electrode 22, the sensing electrodes 24 sense a twisting motion of the freestanding portion 16, referred to as the Coriolis effect, about the axis of symmetry of the freestanding portion 16 (i.e., parallel to the legs of the freestanding portion 16). Because the twisting motion is more readily detectible along the parallel legs of the freestanding portion 16, the sensing electrodes 24 may be positioned along the entire lengths of the legs. The degree to which the freestanding portion 16 twists (deflects) during a vibration cycle as a result of the Coriolis effect can be correlated to the mass flow rate of the fluid flowing through the tube 14, while the density of the fluid is proportional to the frequency of vibration at resonance.
Notable advantages of the Coriolis microfluidic device 10 include the miniaturized scale to which it can be fabricated and its ability to precisely analyze very small quantities of fluids. In FIG. 2, the device 10 is schematically shown as enclosed by a capping wafer 30 to allow for vacuum packaging that further improves the performance of the device 10 by reducing air damping effects. In addition, FIG. 2 shows a getter material 34 placed in the enclosure to assist in reducing and maintaining a low cavity pressure.
The microfluidic device 10 represented in FIGS. 1 and 2 and disclosed in Tadigadapa et al. can be used in a wide variety of applications, as evident from commonly-assigned U.S. Pat. Nos. 6,637,257, 6,647,778, 6,932,114, 7,059,176, 7,228,735, 7,263,882, 7,354,429 and 7,437,912, U.S. Published Patent Application Nos. 2004/0171983, 2005/0126304, 2005/0284815, 2005/0235759, 2006/0211981, 2007/0151335, 2007/0157739, 2008/0154535, and pending U.S. patent application Ser. Nos. 12/031,839, 12/031,860, 12/106,642 and 12/143, 942. As particular examples, U.S. Pat. No. 7,263,882 teaches that chemical concentrations, including those of fuel cell solutions, can be measured by sensing changes in fluid density as a fluid sample flows through a microchannel within a resonating tube of a Coriolis-based microfluidic device, and U.S. Published Patent Application No. 2007/0157739 teaches the capability of detecting potential measurement errors attributable to second phases such as gas bubbles in a fluid being evaluated by a resonating tube of a Coriolis-based microfluidic device. A particular example disclosed in 2007/0157739 is a fuel cell power generation process, during which carbon dioxide, air, and other gases generated or dissolved in a fuel cell solution may form bubbles that can cause errors in chemical concentration outputs based on density. As solution, 2007/0157739 teaches modifications to the construction and operation of the Coriolis-based fluid sensing device of Tadigadapa et al. to promote the detection of second phases such as gas bubbles in a fuel cell solution.
While capable of detecting the presence of gas bubbles in a liquid, Coriolis microfluidic devices of the type taught by Tadigadapa et al. have limited capability for measuring the volume of gas bubbles present in a liquid or the flow rate or density of a gas or gas mixture. In particular, while U.S. Pat. No. 7,263,882 and U.S. Published Patent Application No. 200/0211981 disclose the use of Coriolis-based microfluidic devices for sensing the mass flow rates and densities of gases and gas mixtures, improvements in the sensitivities of such devices are necessary to fully realize the capabilities of such devices.